The present exemplary embodiment relates to a method for producing a rubber including silica and natural rubber, and the rubber produced thereby. It finds particular application in conjunction with guayule rubber, and will be described with reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other similar natural rubber materials. It finds particular suitability for use in association with tires and will be described in association therewith. However, it is to be appreciated that the present exemplary embodiments are also amenable to other applications, such as engineered product applications such as hoses and belts.
Natural rubber, derived from the plant Hevea brasiliensis is a core component of many consumer goods, including medical devices and tires. The United States has a strong reliance on natural rubber, primarily because synthetic alternatives cannot match the high performance properties of natural rubber required for many applications and tend to be expensive. Hevea brasiliensis rubber is a critical material for all types of tires but especially for heavy truck and aircraft tires. These types of tires have a very large percentage of natural rubber because of the low heat buildup of natural rubber based compounds. Similarly, passenger tires may advantageously include a significant amount of natural rubber in specific components such as sidewall and bead filler to provide low heat build-up and flex fatigue resistance.
Over 90% of the Hevea-derived natural rubber imported by the United States originates in Indonesia, Malaysia and Thailand. Natural rubber sources in these countries are under intense threat from potential diseases and blights due to the genetic similarity of the rubber plants. Furthermore, the crop is limited to a restricted geographic area and labor-intensive harvesting methods. In addition, the Southeast Asian natural rubber crop contains many protein contaminants which are responsible for Type-I latex allergies, which are estimated to affect as many as 20 million Americans. The high cost of importation to the United States, as well as the potential for the entire crop to be wiped out by disease and the ubiquity of latex allergies, make non-allergenic domestic natural rubber alternatives particularly attractive.
Accordingly, attention is being directed to the production of natural rubber from plants such as guayule (Parthenium argentatum) and Russian dandelion (Taraxacum kok-saghyz), which yield polymeric cis 1,4-isoprene essentially identical to that produced by Hevea rubber trees in Southeast Asia. A major difference between Hevea rubber (HR) and guayule rubber (GR) is in the amount and types of proteins contained in each species. Hevea rubber possesses many different types of proteins with a 14-kDa “rubber elongation factor” and a 24-kDa “small rubber particle protein” (SRPP) dominating. Both of these are known allergens. In guayule, there are few proteins. A 53-kDa monoxygenase P450 (an allene oxide synthase) comprises about 50% of the rubber particle protein. (See M. Whalen, C. McMahan and D. Shintani, “Development of Crops to Produce Industrially Useful Natural Rubber, Isoprenoid Synthesis in Plants and Microorganisms: New Concepts and Experimental Approaches, pages 329-345, T. Bach and M. Rohmer eds, 2013; the disclosure of which is herein incorporated by reference). Thus, although the chemical structure of the rubber in both species are similar (cis-1,4 polyisoprene), the overall composition of the rubbers is not the same. A comparison of three different types of rubber—Hevea, Guayule, and TSK “Dandelion rubber” is shown in the table below.
TABLEPresence in rubber particlesProtein sequencesRussianFunctionHeveaGuayuledandelionRubber-associatedSmall rubber particle protein+−+Rubber elongation factor+−+Allene oxide synthase−++Defense- or stress-relatedLipoxygenase, chitinase, PGase inhibitor−++Proteases, protease inhibitors−++Phospholipase C, lipases, peroxidase,+−+acid phosphataseDehydration-, wound-, stress-inducible+−−proteinsAnnexin+−+Endomembrane-associated+++Mitochondria-associated−++
The use of silica as a filler in tire rubbers has grown tremendously since its introduction into main line tires in the early 1990's. It has particularly been used in association with synthetic styrene-butadiene and polybutadiene rubbers. This is due to up to a 30% reduction in rolling resistance and up to a 15% increase in wet adhesion in poor weather conditions when a silica filler is used instead of carbon black. To achieve these advantages, it is beneficial to react the silica with a silane coupling agent either before it is mixed into the rubber (an ex-situ process) or during the mixing of the rubber with the silica (in-situ process). Almost all commercial tire compounds using silica use the in-situ process for preparation of silica compounds.
The in-situ silica/silane technology has not been widely adopted in natural rubber compounds due to problems with rolling resistance, wear, handling and tear strength. This may be due to the presence of non-rubber materials in the natural rubber not present in the styrene-butadiene or polybutadiene rubber used in passenger tires. It may also be that these materials, especially the proteins and protein metabolites, may interfere with the silanization reaction to reduce overall physical properties and adversely affect processing. Investigators have tried to overcome these shortcomings by using pretreated (ex-situ) silica such as the Agilon family of products (see Justin Martin and Timothy Okel—“Bringing innovation to the surface: Functionalized slicas for improved natural rubber truck tire vulcanizates”—Technical paper at 184th Rubber Division meeting Oct. 8, 2013) in natural rubber compounds, but this is an extremely costly solution. There remains a need to be able to improve the incorporation of silica/silane into natural rubber compounds using conventional in-situ silica mixing technology.
Processing of rubber components is an important part of making tires. Ideally, very high molecular weight polymers incorporating various functional groups could be used in compounds to achieve maximum performance, but in many cases it is difficult to mix these types of materials. There is always a tradeoff between tire performance and the ability to mix the desired ingredients. There are several ways to measure the processability of compounds. One of the more common techniques is to measure the compounds Mooney viscosity. For the most part, the lower the compound Mooney viscosity, the better the processability. A less common approach, but one that can be applied to more tire building operations is capillary rheometery. In this test, a rubber sample is subjected to various shear rates, and a plot of polymer viscosity versus shear rates is obtained. Since different plant operations operate at different shear rates, the plots generated from the capillary rheometer can be used to predict performance for all of the plant operations, including extrusion, mixing and calendaring.